As microcomputer customers require more user selectable functions on a microcomputer, methods must be found to include these user selectable functions without increasing the cost of the microcomputer. Using pins as control inputs is one way to allow more user selectable functions on a microcomputer. However, merely increasing the number of pins to add user selectable functions is generally not a feasible solution because it usually increases the cost of the microcomputer integrated circuit by increasing the cost of the package.
A significant portion of the cost of a microcomputer integrated circuit is the package. With most package types, the more pins the package has, the higher the cost of the package. Also, there are certain standard packages which are capable of accommodating only up to a certain maximum number of pins. If the microcomputer requires more pins than can be accommodated by that standard package, another generally more expensive package must be used. For example, a particular standard package may have one hundred and twenty-eight pins. A microcomputer that requires one hundred and twenty-nine pins cannot use that standard package because it does not supply enough pins. Thus a larger, more expensive package must be used.
A less expensive solution than using more pins is to multiplex different functions on the same pin. But there is a limit as to how many different functions can be multiplexed on a pin before the different functions start interfering with each other. For example, one pin can serve a dual purpose by having one function during power up and having another function during normal operation. But a third function cannot be added if it would interfere with either the first or the second function already performed by the pin. New ways of multiplexing multiple functions on existing pins are needed.